Reading the intervals: PA, AH and HV

In plain words

A road trip with checkpoints. Each heartbeat starts at the sinus node, crosses the atria, stops at a toll booth (the AV node, which slows traffic on purpose), crosses a bridge (the His bundle), then spreads out through the fast highways of the ventricles. Each interval times one leg of that trip. (A comparison to help it stick, not a technical description.)

What you'll see and do

1. Where the signals come from. At the start of a study, catheters are placed in four standard spots: the high right atrium (HRA), across the His bundle, in the coronary sinus (CS) and at the right ventricular apex (RVA). The His catheter is the one to watch for the intervals. On its channel you'll see three signals in a row:

A (the atria firing) → H (the His bundle firing) → V (the ventricles firing)

2. The baseline intervals are measured first, during the patient's own sinus rhythm, before any pacing starts. They show how healthy the conduction system is to begin with.

The His signal itself is short: normally under 30 ms from start to end.

3. The AV node changes speed, and that's normal. The AH interval is very sensitive to the body's nervous system (autonomic tone). It can stretch or shrink with things like sedation, pain or anxiety. The AV node also slows down on purpose when it's pushed faster. That property is called decremental conduction, and it protects the ventricles from very fast atrial rates. You'll see it later in the course, when the team paces faster and faster and the AH gets longer until a beat drops.

4. The HV is the one that can change the plan. A long HV means the problem is below the AV node, in the His-Purkinje system. That kind of disease may need a permanent pacemaker, so a long HV is a finding the physician will talk about.

5. How it ties back to the ECG. The PR interval on the surface ECG covers this whole trip from the atria to the ventricles. PA, AH and HV are the PR interval broken into its legs, which is how the study finds where a delay is.

6. Sneak preview: the AH "jump." Some people have two routes through the AV node, a fast one and a slow one. That's the setup for AVNRT. During testing, an early beat can block in the fast route and switch to the slow one, and the AH suddenly lengthens by more than 50 ms. That's called an AH jump.

7. Your role. As a new person, start by learning to find A, H and V on the His channel and to follow along as the recording tech measures each interval. Ask to measure a few yourself and compare.

Words you'll hear

• HRA, His, CS, RVA: the four standard catheter spots.
• A, H, V: the atrial, His and ventricular signals on the His catheter.
• Baseline intervals: PA, AH and HV, measured in sinus rhythm before pacing.
• Autonomic tone: the nervous system's push and pull on heart rate and AV-node speed.
• Decremental conduction: the AV node slowing down as it's pushed faster.
• Infranodal (infra-Hisian): below the AV node or His bundle.
• ms: milliseconds (1,000 in a second).

Safety must-knows

1. The intervals are a baseline. They're measured before pacing; know where they're recorded so the team can compare later.
2. If you see dropped beats or a sudden change in rhythm on the monitor, say it out loud.

Ask your preceptor

• How does our recording system label the channels, and which one is the His?
• Who measures and documents the baseline intervals here?
• Can I practice marking A, H and V on a saved recording?

Check yourself

1. Which interval times conduction through the AV node?

1. PA
2. AH
3. HV
4. QT

2. What is the normal HV interval?

1. 5–10 ms
2. 35–55 ms
3. 120–200 ms
4. 300–400 ms

3. The AH interval is especially sensitive to which influence?

1. Body temperature
2. Hemoglobin level
3. Autonomic tone
4. Breathing rate

Concepts only. Always follow your lab's policies and your preceptor's direction.

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